Peritoneal dissemination is the most frequent type of recurrence in patients with gastric cancer with serosal exposure, irrespective of whether they have undergone curative gastrectomy. The purpose of this study was to establish a method to detect micrometastatic cells in the abdominal cavity and predict peritoneal recurrence in patients with such gastric carcinomas. A total of 86 patients with gastric carcinoma, undergoing gastrectomy, were examined. Reverse transcriptase-polymerase chain reaction (RT-PCR) assay was used to detect carcinoembryonic antigen (CEA) mRNA in abdominal lavage fluid. Twenty-four cases without serosal exposure were negative, while all 13 cases with macroscopic peritoneal dissemination were positive for CEA mRNA. Among the 49 cases with macroscopic serosal invasion and without peritoneal metastasis, cancer cells were detected in 27 cases with RT-PCR while in only 6 cases with conventional cytology. All cytologically-positive cases were also positive for CEA mRNA. Among the 27 CEA-positive cases, 15 patients (56%) relapsed with peritoneal metastasis within 12 months after gastrectomy. In contrast, none of the 22 CEA-negative cases had peritoneal recurrence within 16-60 months of observation, whereas in 43 cytologically-negative cases, 10 patients relapsed with peritoneal recurrence. As compared with conventional cytological examination, this method would be clinically more beneficial for detecting free cancer cells in the peritoneal cavity and for predicting peritoneal recurrence in gastric carcinoma with serosal invasion.